This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-60459 filed on Mar. 6, 2000.
1. Field of the Invention
The present invention relates to an air conditioning system that carries out a protective control operation of a compressor when a high pressure in a refrigerant cycle of the air conditioning system becomes abnormally high, to prevent a failure of the compressor.
2. Description of Related Art
In a known type of refrigerant cycle system having an accumulator, if a compressor is operated while a high pressure in the refrigerant cycle system is abnormally high, the compressor will fail. To prevent the failure of the compressor, a first protective value is set near an upper tolerable pressure limit of the compressor. When the high pressure of the refrigerant cycle system exceeds the first protective value, the compressor is forcefully turned off.
Furthermore, a second protective value that is lower than the first protective value is set to prevent the high pressure from reaching the first protective pressure. When the high pressure exceeds the second protective value, a current rotational speed (capacity) of the compressor is maintained or reduced. Specifically, the second protective value is split into a low-pressure side second protective value and a high-pressure side second protective value. When the high pressure in the refrigerant cycle system exceeds the low-pressure side second protective value, the current compressor rotational speed is maintained. When the high pressure exceeds the high-pressure side second protective value, the compressor rotational speed is reduced. However, in the refrigerant cycle system, each one of the protective values is always the same regardless of operation time of the compressor. This causes the following problem at startup of the compressor.
That is, at the startup of the compressor, the compressor rotational speed increases very rapidly from zero to a target rotational speed, and thereby the high pressure in the refrigerant cycle system also increases very rapidly.
Also, at the startup of the compressor, gaseous refrigerant remained in a condenser near an outlet of the condenser is discharged from the outlet of the condenser without completely dissipating its heat. Thus, a gas to liquid ratio of refrigerant at the outlet of the condenser increases. In the refrigerant cycle system having the accumulator, a gas-liquid separator (receiver) that separates refrigerant into gas refrigerant and liquid refrigerant is not arranged between the condenser and a decompressor. As a result, refrigerant discharged from the outlet of the condenser is not separated into the gas refrigerant and the liquid refrigerant before entering into the decompressor. Thus, when the gas to liquid ratio of refrigerant is increased at the outlet of the condenser, a throttle degree of the decompressor increases, and thereby a high-pressure side refrigerant pressure of the refrigerant cycle system rapidly increases.
As a result, as shown in FIG. 10, the high pressure exceeds the second protective values (HPV2, LPV2) at the startup of the compressor. In FIG. 10, PV1 indicates the first protective value, HPV2 indicates the high-pressure side second protective value, and LPV2 indicates the low-pressure side second protective value.
When the high pressure exceeds the second protective values, the current compressor rotational speed is maintained or reduced. However, due to the rapid increase of the high pressure, the high pressure may also exceed the first protective value (PV1), causing forceful shutdown of the compressor. If this happens, the compressor needs to be restarted, and the restart of the compressor disadvantageously requires a certain amount of time.
Furthermore, when the high pressure exceeds the low-pressure side second protective value (LPV2) and then the high-pressure side second protective value (LPV2), the compressor rotational speed is forcefully reduced. In this way, the high pressure decreases below the high-pressure side second protective value (HPV2). At this time point, the current compressor rotational speed is maintained to keep the high pressure between the high-pressure side second protective value (HPV2) and the low-pressure side second protective value (LPV2). However, the compressor rotational speed decreases at a maximum rate. Thus, the high pressure may continue to decrease and thereby may become lower than the low-pressure side second protective value (LPV2). In such a case, since the current operating condition of the air conditioning system has not been changed and thereby still causes the high pressure to increase. Thus, the high pressure may increase once again, and start performance of the compressor is deteriorated.
The present invention addresses the above-described problems. Thus, it is an objective of the present invention to provide an air conditioning system having a refrigerant cycle system, which improves starting performance of a compressor while protecting the compressor.
To achieve the objective of the present invention, an air conditioning system has a compressor protective control unit having first and second protective values. The first protective value is set for preventing a failure of a compressor. The second protective value is lower than the first protective value in order to prevent a high pressure in a refrigerant cycle system from reaching the first protective value. The compressor protective control unit maintains or reduces a capacity of the compressor when the high pressure in the refrigerant cycle system exceeds the second protective value. The compressor protective control unit turns off the compressor when the high pressure exceeds the first protective value. The compressor protective control unit sets the second protective value such that the second protective value used before an elapse of a predetermined time period from startup of the compressor is lower than the second protective value used after the elapse of the predetermined time period. Accordingly, the air conditioning system improves start performance of the compressor, while protecting the compressor.